Why Does an Ammeter Have Higher Internal Resistance at Lower Current Ranges?

[StonieJ]

We recently discovered that the internal resistance of an ammeter when using the 30 mA range is significantly larger than the internal resistance when using its 150 mA range. Despite the fact that I know this is correct, it seems counterintuitive. I would think that since an ammeter on the 30 mA range is used to measure small currents, that a larger internal resistance would effect the given reading too much. On the other hand, a large internal resistance on the 150 mA scale will not affect the measured value as much because the current value is large to begin with. (This probably doesn't make sense, but I can't word it any other way at the moment.) The only thing I can think of is that the larger internal resistance for the 30 mA scale is a sort of protection mechanism that will keep the ammeter from being destroyed if too high a current is accidentally run through it, giving the user time to turn off the current.

 

[Astronuc]

For a voltage input, the larger internal resistance produces the smaller current, but virtue of I = V / R.

So, for a smaller range of I, one needs a larger R for a given V.

 

[chroot]

The larger resistance does indeed affect the low-current circuit more than the high-current circuit. This is bad, of course, but not easily preventable.

The reason it's so is usually because the ammeter is actually just measuring the voltage across a small-value resistor, and using that voltage to drive the needle (or digital display, or whatever). It needs a "large" resistance when dealing with smaller currents, so the voltage across that resistor will be large enough to drive the needle. On the other hand, when dealing with large currents, it only needs a small resistance to generate the necessary voltages to drive the needle.

- Warren